This invention relates to frequency shift keying (FSK) modulators, and more particularly, to FSK modulators having the ability to change from one frequency to another in a quasi continuous manner using digital techiques.
Former FSK modulators instantaneouly changed from one output frequency to another when the input data changed state. For example, if a mark (1200 Hz) was being transmitted and a change to a space (2200 Hz) was desired, the frequency shift took place over a very short period of time compared to the period of the data bit. Due to this rapid and abrupt frequency transition the excessive and undesirable sideband energy was generated.
This invention provides a digital method to smoothly change from one modulation frequency to another over a significant portion of the bit period which reduces the unwanted sideband energy. Eliminating undesirable sideband energy allows more energy to be concentrated in the two primary sidebands. In any application where the amount of power transmitted is limited this reduction of unwanted sideband energy greatly increases the detection capability of the transmitted signal and also reduces any filtering requirements.
Some prior art FSK modulators do produce a smooth mark to space frequency change by the use of linear circuitry. Linear circuitry however has a number of inherent disadvantages. The linear circuitry apparatus requires a number of tuned circuits. These tuned circuits require initial adjustment and periodic callibration. As temperature and other environmental conditions change the output frequency of the linear circuit apparatus is altered.
The time rate change modulator of the present invention is all digital. No adjustment or periodic callibration is required. The apparatus uses a single crystal controlled oscillator to obtain stability of a very high order with a modest component cost. Due to the all-digital design, the time rate change modulator can readily be constructed on a single integrated circuit chip.